Last time I hired a banca was in Puerto Galera. I'm told it was the father of the owner of the cottage where we were staying who took us. Long time since I've been to Boracay.
If your GF doesn't know anyone then you could either go down to the beach and chat to the locals or get your resort to organize it. The resort may charge more, but the boat/skipper may be more reliable. Prices seem to be only partially negotiable - no Trade
Practices Act in the Philippines. I've found it's not a good idea to bargain too hard - maybe find the lowest price
then agree to pay a bit more?
In theory you could go the same day but in practice things move a bit slower in the Philippines, the people you're dealing with will only have a few contacts (relatives?) and it'll take time for them to get their act into gear
. Best to allow at least 24 hours.